The present invention is directed to data networks and in particular to wiring concentrators employed in them.
FIG. 1 depicts a local-area network 10 of a type that can implement the Fiber Distributed Data Interface ("FDDI") standard developed by Committee X3T9.5 of the American National Standards Institute ("ANSI"). The network comprises a plurality of network stations 12, 14, 16, and 18 connected by fiber-optic cables. (Actually, networks that implement all aspects of the FDDI specification except for those dealing with the lowest, physical-medium-dependent sublayer have been implemented without fiber-optic cables or optical signals, and those skilled in the art will recognize that such networks can be considered FDDI networks for the purposes of the present invention. To make the description concrete, however, we will refer to the signal medium as optic fibers.)
In accordance with one topology that the standard specifies, two rings interconnect the stations. A primary ring includes fiber-optic cables 20, 22, 24, and 26, and a secondary ring comprises fiber-optic cables 28, 30, 32, and 34. The signals propagating over the rings convey data organized into "packets," which include information concerning, among other things, the source of the packet and its intended destination. In ordinary operation, a station such as station 12 receives packets from the primary ring by means of a primary input port 36, and a primary output port 38 forwards each packet thus received unless station 12 was itself the source of the packet, in which case station 12 "removes" the packet by not forwarding it. If the packet lists station 12 as its destination, on the other hand, that station still forwards the packet, but it also copies it for use by whatever higher-level entity that station includes.
Also passed along the ring is a special message called a "token," whose receipt enables a station to originate packets rather than just forwarding them. If a station needs to originate a packet, it responds to receipt of the token by removing it rather than forwarding it. It thereby prevents other stations from originating packets until it has finished sending its own packet and returned the token to the ring.
Ordinarily, therefore, the packets follow a primary-ring path that includes dashed lines 40. But this type of operation requires that all stations between the source and recipient--and, indeed, all other stations, too--properly forward the data packets, so if a ring of this type has a large number of stations, it tends to be unreliable in the absence of the secondary ring. While the primary ring carries packets in one direction--i.e., counterclockwise in FIG. 1--the secondary ring carries them in the opposite direction. If station 12 detects no faults in its communications with its immediate neighbors, it merely forwards all secondary-ring packets by means of secondary input and output ports 42 and 44. And if all stations and links are operating properly, the secondary ring carries only so-called "line state" messages rather than packets of the data whose transmission is the network's ultimate purpose. As a result of the secondary ring's reverse direction, however, a station can preserve network integrity by "wrapping" packets from the primary ring back onto the secondary ring when it detects that the link to its downstream neighbor station has been lost.
FIG. 2, for instance, depicts the configuration that results if a break occurs in the link comprising cables 22 and 30. Dashed lines 46 represent the new loop, in which stations 16 and 18 have detected the break and have "wrapped" the loop back upon itself so that station 12, for instance, is interposed in the same loop at two points, rather than at one point in each of two loops. With this arrangement, each station can still communicate with all of the others.
A similarly graceful adjustment occurs if power is lost at one station, as FIG. 3 shows. In that drawing, dashed lines 48 depict the new loop, in which stations 14 and 18 do the "wrapping," while station 12 forwards data in both directions. In this situation, all of the powered stations can still communicate with each other.
In order to arrive at the appropriate configuration, of course, the stations must engage in some type of initialization in which they assess their abilities to communicate with their respective immediate neighbors. The manner in which this initialization proceeds is known to those skilled in the art and is detailed in the FDDI PHY specification. Briefly, however, it involves elements depicted in FIG. 4.
A MAC (Media-Access Control) module 50 in FIG. 4 implements functions of the media-access sublayer of the International Standards Organization (ISO)/Open System interconnection (OSI) reference model's data-link layer. Specifically, it recognizes tokens and monitors the source and destination fields in packets received or forwarded over the primary ring so that it can determine when to originate, copy, and remove packets. The MAC module 50 transmits data by applying (typically four-bit-wide or eight-bit-wide) MAC signals to the MAC-level input port 52 of a PHY module 54, which implements the ISO/OSI physical layer's physical sublayer.
A "4b/5b" encoder 56 encodes the incoming MAC signal four bits at a time into corresponding symbols of five serial bits in accordance with a code that ensures that a transition occurs in the resulting serial signal at least once every three bit times. This encoding occurs at a dock rate set by a local clock 58, and the PHY module 54 provides the resultant PHY output signal at a PHY-level output port 60. A PMD module 62, which implements the ISO/OSI physical layer's PMD (Physical-Medium-Dependent) sublayer, receives the PHY signal at its PHY-level input port 64. An optical transmitter 66--i.e., an electrical-to-optical converter--converts that electrical PHY signal into an optical PMD signal and transmits that signal over fiber-optic cable 20 from (PMD-level) output port 38.
The PMD module 62 also includes input port 42, at which light signals received from the secondary ring over fiber-optic cable 28 are applied to an optical receiver 72. Receiver 72 is an optical-to-electrical converter, which converts the optical PMD signals into electrical PHY signals and transmits the resultant PHY signals from the PMD module's PHY-level output port 74 to the PHY-level input port 76 of the PHY module 54.
The PHY signals received at PHY-level input port 76 have been transmitted from a different station and timed by a different reference clock, which is extremely close in frequency to that of the local dock 58 but nonetheless independent of it. In order to extract data from the received PHY signal, therefore, a clock signal implicit in the PHY signal must be recovered. This is part of the function of the clock-and-data-recovery circuit 78.
The clock-and-data-recovery module 78 typically includes a variable oscillator whose output determines when the incoming signal will be sampled to recover its data. The oscillator phase and frequency are controlled by a phase-locked loop, which in essence compares the transition times of the oscillator output with those of the incoming PHY signal and adjusts its frequency to maintain a predetermined relationship between them. This is the main reason for the 4b/5b code that the PHY module employs: without such a code, which insures that the PHY signal never goes for more than a maximum time without producing a transition, the phase-locked loop could lose synchronization with the incoming signal, and the clock-and-data-recovery unit 78 would then sample the PHY signal at the wrong times and thereby extract the data incorrectly.
The clock-and-data-recovery circuit 78 applies the data thus recovered, together with the oscillator output of the phase-locked loop, i.e., the recovered clock, to a framer and elasticity buffer 80. The local, free-running clock 58 clocks all operations downstream of circuit 80, and circuit 80 provides buffering and adds or removes spacer symbols to accommodate the difference between the local clock rate and the recovered clock rate. It also divides the received data-bit stream into five-bit frames, which it applies to a decoder 82.
Decoder 82 reverses the operation performed by encoder 56 and presents the results at the PHY module's MAC-level output port 83. In the illustrated, normal mode, port 83 forwards the results to the MAC-level input port of a similar PHY module 84 for forwarding along the secondary ring. In one of the wrap modules, the MAC module 50 receives the resultant MAC signal and forwards or otherwise processes it in accordance with normal ring operation. Of course, a higher-level entity not shown in the drawing will typically apply data to the MAC module 50 for inclusion in packets that the MAC module 50 originates and/or receive the data contents of packets that the MAC module copies.
The foregoing discussion illustrates that station 12 uses PHY and PMD modules 54 and 62 collectively to communicate with the rings at primary output and secondary input ports 38 and 42 of FIG. 1. It uses similar PHY and PMD modules 84 and 85 to communicate at ports 36 and 44. In the normal, forwarding configuration described above, the MAC module 50 forwards MAC signals from PHY module 84 to the MAC-level input port 52 of PHY module 54, ultimately for transmission over fiber-optic cable 20 to the next station in the ring. Before this configuration is adopted, however, an initialization process must occur that, among other things, synchronizes clock-and-data-recovery module 78 with the upstream station's local dock and establishes that the PHY and PMD modules 54 and 62 can communicate in both directions with corresponding circuitry at station 18. This is among the tasks of an SMT (Station Management) module 86.
The SMT module is a control circuit that determines, among other things, whether the station is to forward packets to the next station or "wrap" them back toward the station from which they came. Among the criteria that it uses are some for which it tests in the initialization operation and in subsequent monitoring. In the initialization operation, the SMT module operates switches SW1 through SW4 to states b, a, a, and a, respectively. That is, it decouples the MAC module 50 from the PHY modules 54 and 84 and decouples those modules from each other. It additionally operates a switch SW5 to connect the optical transmitter to a generator 87 of "line state" messages specified in the FDDI protocol.
The protocol specifies a sequence of such messages, and the SMT operates generator 87 to send them from the primary output port 38.
Station 18 uses these signals to synchronize the phase-locked loop in its clock-and-data-recovery module with station 12's local clock 58. It simultaneously transmits signals over the secondary ring back to station 12 of FIG. 4, which the clock-and-data-recovery module 78 employs for synchronization purposes. That is, the SMT module 86 monitors the operation of the clock-and-data-recovery unit 78 by, for instance, observing its phase-locked loop's error signal and thereby determining when it has reached synchronization.
With synchronization thus achieved, each message in the sequence is sent until, by operation of a line-state-detector 88, the SMT module 86 detects reception of the same message at port 42 from its counterclockwise neighbor 18, after which it proceeds to send the next line-state message in the sequence.
The SMT module 86 simultaneously operates the other PHY module 84 to conduct a similar colloquy with its clockwise neighbor 14, and, when both PHY modules have completed all messages in the prescribed line-state sequence, the SMT operates switches SW1 through SW5 to the states depicted in FIG. 4, in which the MAC module 50 monitors signals received at port 36 from the primary ring, and station 12 forwards signals on both rings in their respective directions except for the packets that the MAC module 50 removes from the primary ring. When the switches are thrown, the "scrub" function must be performed to remove any data packets on the ring which might have been corrupted by the changing switches. This is done by one of several methods outlined in the FDDI SMT specification.
If, as is typical, the initialization colloquy with one neighbor is completed before that with the other, the switches assume the appropriate wrap configuration until the other initialization is completed. If initialization with the counterclockwise neighbor 18 finishes first, for instance, switches SW1 through SW5 assume states a, b, a, a, and b, respectively, to enable the MAC module 50 to monitor signals that have been received at the secondary-ring input port 42 and will be forwarded, if the MAC module 50 does not remove them, from the primary-ring output port 38.
Of course, defects can arise not only before initialization but also afterward, while the ring is in steady-state operation. During such operations, therefore, the SMT module monitors the PMDs for the QLS line state (loss of carrier), the clock-and-data-recovery module for loss of phase lock, and the decoder 82 for improper code sequences. When errors meeting predetermined criteria occur, the SMD operates switches SW1 through SW4 to reconfigure the station accordingly, and it transmits the QLS line state, which causes ring operation to be reinitialized.
The dual-ring organization is not the only topology of which the FDDI standard admits. Stations of the type depicted in FIGS. 1, 2, and 3 are referred to as "dual-attachment" stations. A dual-attachment station provides two interfaces, one for each of its two neighbors. This arrangement is necessary if, as is often the case, the stations are remote enough from each other that it is not practical for a common circuit to monitor their operations and form data pathways around defective links or stations that are not operating properly. On the other hand, if a number of stations can indeed be connected to such a common circuit, they can be implemented as "single-attachment" stations, each of which provides only a single interface to this common circuit.
FIG. 5 depicts such a topology. In FIG. 5, each of a plurality of stations 92, 94, 96, and 98 communicates with each of its neighbors by only a single, single-direction line. For ongoing operation, this is adequate to provide the required functions: the packets travel the ring, being forwarded by each station to its downstream neighbor, which generates, copies, or removes packets in the manner described above. To provide the necessary reliability, however, a concentrator 100 is interposed in all of the links so as to monitor them and determine whether they are functioning properly. If so, the ring is that formed by the fiber-optic cables 102 connected as indicated by the dashed connection lines 104 in FIG. 5. The concentrator 100 includes provisions for rerouting ring signals around a defective link or station, however, and it thereby provides the reliability that the networks of FIGS. 1, 2, and 3 provide by way of the dual ring.
In some networks, only subsets of the stations on a given network may lend themselves to interconnection by concentrators, so the concentrators themselves may act as dual-attachment stations on a dual ring. FIG. 6 depicts such a concentrator 110. Concentrator 110 in FIG. 6 includes PMD modules 112 and 114 as well as PHY modules 116 and 118 in the conventional arrangement of a dual-attachment station. That is, signals from a cable 120 on the secondary ring are ordinarily simply forwarded to the next link 122 in the secondary ring, while those received from link 124 on the primary ring are "read" by MAC modules to determine whether they should be copied and whether they should be forwarded over the next link 126 in the primary ring.
This MAC-level activity is performed by MAC modules in the single-attachment stations ("SAS1, SAS2, . . . ") 128, 130, 132, and 134 connected to concentrator 110. In principle, therefore, concentrator 110 does not itself need to include a MAC module, as it does PHY and PMD modules. In practice, however, it ordinarily will include a MAC module 138; the concentrator 110 will usually include additional circuitry, not shown, for transmitting and receiving status and configuration information necessary for proper network maintenance, and such a higher-level entity would communicate over the network by means of the MAC module 138.
As was stated above, initialization requires full-duplex communication between neighboring stations. Like stations 92, 94, 96, 96, and 98, however, single-attachment stations 128, 130, 132, and 134 are not capable of performing such fall-duplex communication with their neighbors; station 130 communicates with station 132 by only a single, one-way channel, and its communication with station 128 is also one-way in nature. By including detail omitted from FIG. 5, FIG. 6 shows circuitry that has heretofore been considered necessary in concentrators such as concentrators 100 and 110 in order to perform the initialization process that the FDDI protocol requires.
Specifically, FIG. 6 shows that concentrator 110 includes not only a PMD module 148, 150, 152, or 154 for each attached single-attachment station but also a PHY module 140, 142, 144, or 146, which is needed for initialization. To determine whether the link formed with station 128, for instance, is operable, an SMT module 156 operates module 140 (by means of connections that FIG. 6 omits for the sake of simplicity) to conduct the required initialization colloquy with station 128 and observe the results. If the link with that station and those with the other stations prove to be operable, the SMT configures concentrator 110 to connect the various PHY modules 140, 142, 144, and 146 as shown. On the other hand, if station 130, for instance, proves to be defective, the SMT will operate a switch matrix 157 to re-route the MAC signals, causing the MAC output of PHY module 140 to be applied to PHY module 144 rather than PHY module 142. Station 128 would thereby replace station 130 as the upstream neighbor of station 132.
A little reflection reveals that judicious use of concentrators can actually increase network reliability over that of a pure dual ring, even though the latter uses only dual-attachment stations. Additionally, a concentrator can reduce individual station cost, since the "FDDI corner" in a single-attachment station can be significantly smaller and less expensive than the corresponding circuitry in a dual-attachment station. Some reduction in cabling cost can also result.
Unfortunately, a significant part of the savings in individual-station cost is illusory when conventional concentrators are used. Although the use of single-attachment stations permits half of the PHY and PMD modules to be eliminated, the effect is simply to "move" those modules to a concentrator.